1. Field of the Invention
The present invention relates to a film formation method and apparatus for a semiconductor process for forming a film of silicon nitride or silicon oxynitride doped with an impurity on a target substrate, such as a semiconductor wafer. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices for constituting semiconductor integrated circuits, a target substrate, such as a semiconductor wafer, is subjected to various processes, such as film formation, etching, oxidation, diffusion, reformation, annealing, and natural oxide film removal. Jpn. Pat. Appln. KOKAI Publication No. 2004-6801 discloses a semiconductor processing method of this kind performed in a vertical heat-processing apparatus (of the so-called batch type). According to this method, semiconductor wafers are first transferred from a wafer cassette onto a vertical wafer boat and supported thereon at intervals in the vertical direction. The wafer cassette can store, e.g., 25 wafers, while the wafer boat can support 30 to 150 wafers. Then, the wafer boat is loaded into a process container from below, and the process container is airtightly closed. Then, a predetermined heat-process is performed, while the process conditions, such as process gas flow rate, process pressure, and process temperature, are controlled.
Conventionally, silicon oxide films (SiO2 films) are mainly used as insulating films for semiconductor devices. However, in recent years, owing to the demands of increased integration and miniaturization of semiconductor integrated circuits, silicon nitride films (Si3N4 films) are used in place of silicon oxide films, as usage. For example, silicon nitride films are disposed as films resistant to oxidation, films for preventing impurity diffusion, and sidewall films of gate electrode structures. Since silicon nitride films provide a low coefficient of impurity diffusion and a good barrier property against oxidation, they are very suitable as insulating films for the purpose described above.
On the other hand, in recent years, increasing the operation speed of semiconductor devices is also an important factor. In this respect, silicon nitride films have a relatively high dielectric constant, which increases parasitic capacitance and thus is problematic. Specifically, with increase in parasitic capacitance, the mobility of electrons is suppressed, so the device operation speed decreases. Further, where a silicon nitride film is used for a sensor of the charge storage type, there is a problem in that parasitic capacitance increases a background level.
Under the circumstances, it has been proposed to dope a silicon nitride film with an impurity, so as to decrease the dielectric constant while maintaining the impurity diffusion coefficient and oxidation barrier property. U.S. Pat. No. 6,815,350 discloses a method of forming a silicon nitride film doped with boron (B) as an impurity, by CVD (Chemical Vapor Deposition). Films of silicon nitride doped with boron (SiBN) have not only a low coefficient of impurity diffusion and a good barrier property against oxidation, but also a very low dielectric constant, so they are very useful as insulating films.
However, silicon nitride films doped with boron, which are formed by CVD, have a relatively low strength and a low etching resistance. As a consequence, for example, when dry etching is performed in a subsequent step, silicon nitride films doped with boron tend to be excessively etched.